Focal width correcting lens system for concentrating sunlight

ABSTRACT

Linearly imaging converging lenses situated in parallel, which track the elevation of the sun around their east-west axis, have a high degree of area usage and are therefore particularly suitable for architectonic integration. They act selectively in regard to the incident solar radiation, because they convert the direct sunlight via receivers situated in their focal lines into useful energy, and illuminate the spaces lying underneath with pleasant diffuse light. Their weak point is that they change the focal width and the lateral displacement of the focal line as a function of the solar azimuth angle. 
     The present invention describes a tracking system which transfers the geometrical conditions into a mechanical coupling system which corrects the focal width and the displacement of the focal lines exactly and simultaneously. The tracking system represents a mechanical analog computer which solves trigonometric equations using nested orbit functions.

In contrast to reflecting optics (concentrating mirrors), refractive optics (lenses) offer several interesting advantages in the concentration of sunlight.

In solar energy exploitation facilities, concentrating optics are advantageously used if either high temperatures are needed for operating thermodynamic machines or processes, or large photon densities are advantageous for photochemical reactions—for example, for the effective exploitation of light-induced processes, such as typically photovoltaic current generation. While the radiation converters (receivers) positioned in the focal planes of the optics, sometimes having quite large dimensions, result in shadowing of a part of the primary incident solar radiation in the case of mirror concentrators, because their focal planes lie in front of the mirrors in the direction of the sun, this negative effect does not occur in principle in lens systems, whose focal planes lie behind the mirrors.

A further advantage of the lens systems is related to the additional usage possibility of the diffuse (hemispherical) component of the solar radiation. In principle, concentrating mirrors or lenses may only unite the direct (parallel) component of the solar radiation in focal points or focal lines, but the usability of the diffuse light, after the reflection on the mirror or the refraction in the lens system, has a significant difference.

While the diffuse light is reflected back diffusely from the mirror unused into the celestial half-space, the light exiting diffusely below the lens toward the terrestrial half-space may be used well for illumination purposes. It is therefore understandable that lens systems for concentrating sunlight are preferably implemented as envelopes or partial envelopes of living spaces. This selective separation of direct and diffuse radiation by lens systems automatically results in climate control of the spaces lying underneath (due to the lower energy input), in addition to the production of processing heat, power or current, and pleasant, nonglare diffuse illumination.

The cited advantages are represented typically in Czech Patent Application SG-1 number 284185 (“Envi”) and in a variation in EP 99955833.1 (inventors: writer of this application et al.). While in the Envi application, Fresnel lenses, which are producible especially cost-effectively (a further advantage of lens technology) directly form a roof or facade surface permanently oriented south, lens systems are described in EP 99955833.1 which, below a transparent protective envelope, move tracked on one or two axes with the solar status.

In the patent application “Envi”, single-axis concentrating, focal line generating lenses are used. Because the longitudinal axes of these lenses are oriented from east to west, the sun is incident at different angles on the lens planes in the course of the day (±90° azimuth). The seasonal elevation differences of the sun also result in an angle variation of ±23.5° during the year. Only this is corrected in the Envi system, in that the linear receivers are shifted upward or downward equidistantly to the entry lens as a function of the elevation angle. The displacement of the focal lines in relation to the lens occurring as a function of the azimuth angle remains uncorrected.

The conditions are different in EP 99955833.1. If the lens tracks the sun in two axes, corresponding to a described embodiment, the receiver always lies precisely in the focal point or the focal line. However—as is advisable for buildings in particular—if the linearly imaging lens is oriented to the south and is only angle-compensated over its east-west axis, high angle variations also occur here during the day because of the azimuth movement of the sun and the focal line is additionally displaced in the longitudinal direction as a function of the azimuth angle.

In addition to the advantages cited at the beginning of lens concentrators in relation to mirror concentrators, the lenses also have a decisive weakness in comparison to converging mirrors. Specifically, while in converging mirrors radiation which is not incident vertically on the mirror entry aperture does lead to a spatial offset, but not to a focal width change, both effects occur in lens systems. The focal width changes result in this case from the fact that the radiation is refracted away from the vertical upon exit from the denser optical medium (lens) into the more diffuse optical medium (air) according to the function

$\frac{\left( {\sin (\alpha)} \right)}{\left( {\sin (\beta)} \right)} = {\frac{\left( c_{1} \right)}{\left( c_{2} \right)} = n}$

Lens systems like the two described may not exploit the potential of these optics with full efficiency, because their radiation receivers are set to a fixed focal width and are thus irradiated at variable radiation strengths as a function of the diagonal incidence of the sun, and/or larger or smaller radiation components no longer even reach these receivers. In addition, energy is lost due to the offset of the focal lines along the receiver axes.

The present invention of a focal-line-correcting lens system is therefore based on the object of correcting the serious flaws listed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a (Fresnel) lens oriented south (x axis) in an x, y, z coordinate system, which is tracked around the east-west axis (y axis) according to the elevation of the sun.

FIG. 2 shows the relationships for a lens having the focal width fα−100. (100=dimensionless length)

FIG. 3 illustrates the geometrical relationships in a Z-Y coordinate system.

FIGS. 4 a, 4 b, and 4 c illustrate an embodiment of the present invention in which the linear (Fresnel) lens is corrected for focal widths (Z axis) and also offset (Y axis) correspondingly, while the absorber pipe located in the focal line is fixed in location.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In FIG. 1, a (Fresnel) lens (1) oriented south (x axis) is shown in an x, y, z coordinate system, which is tracked around the east-west axis (y axis) according to the elevation of the sun. The focal width of this lens varies as a function of the angle α, which is predefined by the daily solar status. For α=0, the focal line has no offset S in relation to the lens in the y direction, while this offset is present at α=70° S, for example. For α=0, the lens has the greatest focal width fx₀, while this is significantly shorter at α−70°, for example.

A simulation of the beam paths on linear lenses while varying the angle a results in the following functional relationships:

f·f·α ₀·cos² α S=f·α ₀·cos α·sin α

FIG. 2 shows the relationships for a lens having the focal width fα−100. (100=dimensionless length)

FIG. 3 illustrates the geometrical relationships in a Z-Y coordinate system. The angle α is plotted from 0°-90° in parallel to the Z axis (for the definition of α see FIG. 1), which is the measure of the focal width variation. A y value is assigned to each angle value of α (z). This y value is equivalent to the displacement of the focal line in the ± direction in relation to the lens (FIG. 1, 1). If one varies the azimuth angle α from +90° to −90°, which corresponds to the solar path equal to day-night, the focal width f and the displacement in the y direction vary according to Table 3 a.

The path curve which a point on the radiation receiver must travel through in the z-y coordinate system to lie exactly in the focal line both in regard to the z plane and also the y plane thus moves on the circumference of a circle whose diameter D corresponds to the maximum focal width of the optic at the angle α=0. The dimensionless length D=100 was selected for the figures.

FIG. 4 illustrates how the geometrical relationships described result in a tracking system according to the present invention for correcting the focal width and the lateral offset of the focal line in the event of non-orthogonal incidence of the sun in the azimuth plane for east-west oriented lenses. In this case, (1) is the lens oriented south, which performs the required corrections in relation to the elevation of the sun around the y axis (3) (east-west). A version of the present invention is illustrated in FIGS. 4 a through 4 c, in which the linear (Fresnel) lens (1) is corrected for focal widths (Z axis) and also offset (Y axis) correspondingly, while the absorber pipe (3) located in the focal line is fixed in location. This embodiment of the present invention is especially advisable if fluids having high temperatures and pressures are generated in the absorber pipe. In order that the absorber pipe located centrally below the lens is always irradiated with the maximum radiation concentration even in the event of changing solar height, the lens (1) is automatically corrected for elevation by rotation around the absorber pipe (3).

The noon position is shown in FIG. 4 a, in which the sunbeams are incident vertically on the lens plane. Two circular disks are identified by (2), which are mounted so they are rotatable around their center points and whose diameter D corresponds to the focal width fα° from FIG. 1, FIG. 3, and Table 3 a. The lens is connected to the circumference of the circular disks at the points 2 a via a pivot eye.

One of the two disks (in the example, arbitrarily the left) is connected via a second pivot eye (4 a), which is also mounted so it is rotatable around 2 a, to a solar direction indicator (4), which may be shifted variable in length by the pivot eye (4 a). The solar direction indicator (4) is permanently connected on the side diametrically opposite to (4 a) to a second pivot eye (4 b). This eye is attached to a rigid frame, which is not connected to the circular disks (2), but is continually adjusted with the lens (1) according to the elevation by rotation around the absorber pipe (3).

A solar direction sensor (5) is attached to the tip of the solar direction indicator (4). This sensor, which is typically equipped with two photodiodes as a “shadow sword” sends control commands to the motor located in the rotational axis of 2 to rotate until both photodiodes receive the same light signal. In the solar noon position shown in FIG. 4 a, the solar direction indicator (4) is then perpendicular to the lens plane (1). The focal width of the lens corresponds to the maximum focal width fα° as required in FIG. 3 and Table 3 a. The displacement (5) also corresponds to the requirement and is 0 in this case, i.e., the focal line lies below the lens in orthogonal projection of the sunbeams and the receiver tube is completely irradiated. However, this requires that the elevation of the lens is also adapted to the geographical latitude and the exact date/time of day. According to the present invention, this may be achieved especially simply if an identical shadow sensor (5 a), which is pivoted by 90°, is assigned to the azimuth sensor (5), which regulates the elevation using the motor (6 a).

The case of the solar irradiation at the azimuth angle 45° (morning position) is shown in FIG. 4 b. In addition, the circular disks have rotated by 90° clockwise. The resulting new focal width fα45° is, in correspondence with FIG. 3 and Table 3 a, D/2. The displacement Sα45° has also reached its maximum at D/2 as well in this position. The fixed absorber pipe is in turn completely irradiated.

The morning limit position at the azimuth angle α=90° is shown in FIG. 4 c. The circular disks have rotated by 180°. As required, the focal width is zero here, i.e., the lens (1) lies on the absorber pipe (3), and the displacement (S) is also 0.

However, this position is an impractical limiting value, because in the event of sunbeams incident horizontally on the lens plane, no light is conducted through the lens structure (total reflection on the surface). In practice, energy may thus only be coupled into the absorber pipe when the azimuth angle α is less than 90°.

The relationships from sunrise to noon illustrated in FIGS. 4 a-4 c are continued mirror-symmetrically to the noon normal line by further 180° rotation of the disks clockwise until sunset. After sunset, the disks are rotated counterclockwise by 360°, so that the entire configuration is again in the starting position the next morning.

In relation to the prior art described at the beginning, the system according to the present invention has the advantage of significantly improved efficiency. The correction of the focal width as a function of the azimuth angle and the correction of the lateral focal line offset may classically be implemented by the use of two independent positioning motors in the Z and Y directions, which receive their control signals via sensors or a digitally stored program.

The novelty of the present invention is that both movement correctors are generated simultaneously by a simple mechanical coupling system based on circular functions. The present invention thus represents a mechanical analog computer for solving trigonometric equations by nested orbits.

The technical embodiment is only described in principle, while maintaining the essential geometric parameters (diameter of the circular disk=fα 0; circular disk adjoins lens plane; solar vector rod attached rotatably on circumference of circle; second point of rotation of the solar vector rod implemented at the distance of the circle radius, independently of the circular disk in the lens plane, in such a way that the solar vector rod may both rotate and also shift longitudinally), the principle may be solved for the technical clients in greatly varying ways.

For example, the circular disks may be replaced by rotating bars rotated around their center points. The rotation around the center points of the circular disks or rotating bars may be implemented by linear traction forces which typically enclose the solar sensor rod (4). Instead of fixing the absorber pipe (3) and shifting the lens (1) along the Z and Y axes, the reversed procedure may also be used; the lens is fixed, while the absorber pipe executes the corresponding movement.

In principle, according to the present invention, while maintaining the kinematics of the solar vector rod, only its Z axis information (focal width adaptation) may also be mechanically transferred to a telescoping tube or an equivalent length-variable device (e.g., folding rod) and the simultaneous correction of the Y axis (focal line offset) may be dispensed with. To ensure complete illumination of the absorber pipe in this case as well, the lenses project beyond the absorber pipe on the right and left by D/2.

The tracking kinematics according to the present invention also function in principle for lenses which are not positioned exactly along the east-west axis. In this case, only the maximum elevation angle has to be increased as a function of this deviation.

The construction may be implemented in manifold ways for the mechanical experts while maintaining the geometrical framework conditions on which the present invention is based. 

1. A tracking system for correcting the focal width and the focal line offset in lenses which are positioned east-west and are tracked in elevation, characterized in that a mechanical coupling system performs both corrections simultaneously.
 2. The tracking system according to claim 1, wherein both sets of position information are given by the path curve which a solar vector rod rotating on the circumference of the circle runs through as a function of time.
 3. The tracking system according to claim 1, wherein the solar vector rod is mounted so it is rotatable around its attachment point on the circular circumference and is longitudinally displaceable and, in addition, is mounted so it is rotatable in a second bearing point which is not connected to the circle.
 4. The tracking system according to claim 1, wherein the diameter of the circle corresponds to the largest possible focal width of the lens in the event of vertical incidence of the solar radiation on the entry plane.
 5. The tracking system according to claim 3, wherein the second bearing point of the solar vector rod lies in the point of intersection which the circle directly adjoining below the lens plane forms with the focal plane.
 6. The tracking system according to claim 1, wherein the circles comprises circular disks or rotating bars rotatable around their center points, whose diameters or lengths correspond to the maximum focal width.
 7. The tracking system according to claim 1, wherein in azimuthal solar direction sensor is located in the end of the solar vector rod facing toward the sun, which provides the control signals for the motorized rotation of the circular disks.
 8. The tracking system according to claim 1, wherein an elevation sensor is additionally located at the end of the solar vector rod, which rotates the entire configuration via a motor into the correct elevation as a function of time.
 9. The tracking system according to claim 1, wherein either the lens is fixed and the strip-shaped receiver is displaced in the Z and Y axes or vice versa.
 10. The tracking system according to claim 1, wherein only the focal width correction is performed via a telescoping tube or an equivalent mechanical configuration by the kinematics of the solar vector rod, while the correction of the focal line offset is performed by lenses extended on the left and right by D/2. 